1. Technical Field
The present disclosure relates to an offset frequency estimation method and apparatus, and more particularly, to an offset frequency estimation method and apparatus for preventing incorrect offset frequency estimation by using adaptive skipping of error samples.
2. Discussion of Related Art
In general, in systems using a modulation method, such as phase shift keying (PSK) or minimum shift keying (MSK), correct estimation of a carrier frequency offset is important.
A conventional offset frequency estimation apparatus receives an input signal comprised of complex samples. The received samples may contain noise generated in a transmission/reception process.
A reference sample is a signal that does not contain a noise component and can be obtained by equalizing and filtering the received samples. The reference sample or the received samples can be conjugated. A signal pre-set in transmission and reception systems can also be used as the reference sample.
For the convenience of description, it is assumed that the reference sample is conjugated. Conventionally, the offset frequency estimation apparatus multiplies the received samples by a conjugate value of the reference sample. However, when the received samples are conjugated, the offset frequency estimation apparatus multiplies the reference sample by a conjugate value of the received samples.
Phases of the received samples varying with time are detected from the result obtained by multiplying the received samples by the conjugate value of the reference sample. The detected phases of the received samples can be converted to an offset frequency using a least squares method.
Equations 1 through 4 show a conventional offset frequency estimation process.R=S*ejωt  (1)
Here, R denotes a received sample ejωt denotes a noise component generated in a signal transmission/reception process, and S denotes the reference sample, which does not contain the noise component.
                                                        X              =                              R                *                                  S                  *                                                                                                        =                              S                *                                  ⅇ                                      jω                    ⁢                                                                                  ⁢                    t                                                  *                                  S                  *                                                                                                        =                              A                *                                  ⅇ                                      jω                    ⁢                                                                                  ⁢                    t                                                                                                                          =                              A                *                                  (                                                            cos                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        +                                          j                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                                        )                                                                                        (        2        )            
Equation 2 shows the case where the reference sample is conjugated.ωt+φ+ε=arctan(sin ωt/cos ωt)  (3)
                              (                                    ω              ⁢                                                          ⁢              t                        +            φ            +            ɛ                    )                ⁢                  →          LS                ⁢                  (                      ω            ^                    )                                    (        4        )            
In the conventional offset frequency estimation process described above, an error may be generated due to error samples in the offset frequency estimation process. Incorrect offset frequency estimation can result in incorrect detection of a reception frequency.